Driver assistance systems generally serve to assist the driver in controlling the vehicle during driving. Such systems can, in particular, be surroundings detection systems that detect foreign objects by means of suitable sensors in order to change the driving behavior of the vehicle correspondingly. In this context, on the one hand, a collision with a detected external object can be prevented or the severity of an accident can be reduced by braking the vehicle suitably. Furthermore, a vehicle that is traveling ahead can be detected and the distance from the driver's own vehicle can be regulated, for example, in the form of an ACC (adaptive cruise control) process or as the vehicle traveling in a convoy mode in a line composed of a plurality of vehicles. In particular, radar-based or ultrasound-based sensors can be used as surroundings detection sensors for detecting distance, and video systems can be used for recording and detecting the objects and, if appropriate, for detecting distance through the evaluation of stereo images. Furthermore, the driver assistance systems may also be systems that detect a critical driving state, for example, a tendency of the vehicle to tip over or a tendency of the vehicle to skid, and that communicate this to the brake control device.
Driver assistance systems are typically implemented separately from the brake control device, and can output what are referred to as external brake request signals to the brake control device of the brake system, in particular, via a data bus. The brake control device therefore carries out a control process of the brakes on the basis of internal brake signals, which may comprise, in particular, the setpoint brake request input by the driver via a brake value signal generator and, for example, internal closed-loop control processes such as ABS or a stability control process, and on the basis of additional external brake request signals.
DE 101 18 708 A1 and DE 101 18 707 A1 describe processes for regulating vehicle speed and for preventing collisions, in which, when a possibly imminent braking intervention is detected, a hydraulic brake system can be biased by increasing the brake pressure of the brake hydraulics without an appreciable braking operation already being initiated. For this purpose, an appropriate flag can be set by a decision unit that also receives a brake request signal S1 of the driver. The actual braking operation can therefore subsequently be carried out more quickly since the corresponding pressure build-up of the brake hydraulics does not have to be initiated first so that time can be gained during the actual braking operation.
Such biasing when a relatively high probability of a collision is detected may be carried out in a hydraulic brake system in a directly internal fashion by means of the control device. However, in the case of pneumatic brake systems of commercial vehicles, surroundings detection systems are generally embodied as external systems that communicate with the brake control device, for example, via a vehicle-internal data bus such as the CAN bus, and by means of suitable protocols. The external request for biasing without a braking effect, or without a relevant braking effect, would therefore require a suitable set of instructions that cannot be implemented with current systems.
In pneumatic brake systems of a commercial vehicle field, external brake requests can be requested by means of various standards. SAE-J1939 describes the logging of such external brake requests, and, accordingly, an external brake request contains information about the priority, the control mode, and the acceleration setpoint value.
The indication of the priority is relevant since, according to this standard, the external brake request signal is continuously output by the external system, with the result that when a necessary braking effect is absent, a low priority is specified; in the case of a priority signal with three stages, the lowest priority can therefore be indicated. The control mode can, in particular, be additive or maximal; in the case of an additive control mode the external brake request is set in addition (additively) to the internally set braking effect, and, on the other hand, in the case of a maximum control mode, formation of a maximum value is brought about, i.e., the highest value of the internal brake request and the highest value of the external brake request are respectively formed. Furthermore, the external brake request signal has an indication about the setpoint acceleration to be set, i.e., the acceleration setpoint value, wherein braking operations correspondingly have a negative value.